Aluminum cap with electroless nickel/immersion gold

ABSTRACT

A method of forming a solder bump (and the resulting solder bump structure), comprising the following steps. A structure having a metal bond pad formed thereover is provided. A patterned cover layer is formed over the structure. The patterned cover layer including an opening exposing a portion of the metal bond pad. The patterned cover layer opening including side walls. A metal cap layer is formed over at least the exposed portion of the metal bond pad and the patterned cover layer side walls. A solder bump is formed over the metal cap layer.

FIELD OF THE INVENTION

The present invention relates generally to fabrication of semiconductordevices, and more specifically to methods of fabricating wafer bumps.

BACKGROUND OF THE INVENTION

Electroless nickel/gold (Ni/Au) under bump metal (UBM) is a commonlyused UBM for wafer bumping used in Flip Chip applications because it's alow cost maskless process. This process involves the dipping of siliconnitride (Si₃N₄)/polyimidecover/benzocyclobutene (BCB) passivated wafersinto a series of plating baths to grow a thick layer of Ni studs (UBM).After this UBM process, the wafer is sent for bumping and reflow to formround solder ball interconnects.

However, the adhesion of this electroless Ni/Au UBM to the passivationlayer is poor, and during plating, the plated Au will follow a capillaryaction and become deposited on the side wall of the PI/BCB passivationlayer. During solder bump reflow, this will cause solder to flow intothe side wall of the passivation layer as Au is proven to be a goodwetting layer of solder. Thus, the solder will attack the underlyingaluminum (Al) pad causing corrosion to the Al pad and affecting thereliability of the package.

U.S. Pat. No. 6,452,270 B1 to Huang describes increased adhesion betweenthe UBM and passivation by using a Ti UBM with a closed-loop shape.

U.S. Pat. No. 6,521,996 B1 to Seshan describes a lower adhesion layerthat provides good adhesion to the passivation layer.

U.S. Pat. No. 6,426,281 B1 to Lin et al. describes a passivation layerthat increases the adhesion to the UBM.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provideimproved methods of forming wafer bumps.

Other objects will appear hereinafter.

It has now been discovered that the above and other objects of thepresent invention may be accomplished in the following manner.Specifically, a structure having a metal bond pad formed thereover isprovided. A patterned cover layer is formed over the structure. Thepatterned cover layer including an opening exposing a portion of themetal bond pad. The patterned cover layer opening including side walls.A metal cap layer is formed over at least the exposed portion of themetal bond pad and the patterned cover layer side walls. A solder bumpis formed over the metal cap layer. The invention further includes asolder bump structure formed from this method.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be moreclearly understood from the following description taken in conjunctionwith the accompanying drawings in which like reference numeralsdesignate similar or corresponding elements, regions and portions and inwhich:

FIGS. 1 to 6 schematically illustrate a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of the present invention forms, inter alia, an additionallayer of metal over the metal bond pad to prevent exposure of the coverside walls as the additional layer of metal covers up the cover sidewalls.

Initial Structure—FIG. 1

FIG. 1 schematically illustrates a structure 10 having a metal bond pad12 formed thereover.

Structure 10 is preferably a semiconductor wafer comprised of silicon orgermanium and is more preferably a silicon semiconductor wafer as willbe used for illustrative purposes hereafter.

Metal bond pad 12 is preferably from about 0.5 to 1.5 μm thick and ismore preferably from about 1.0 to 1.5 μm thick; and is preferablycomprised of aluminum (Al) or AlSi and is more preferably aluminum (Al)as will be used for illustrative purposes hereafter.

Formation of Patterned Cover Layer 14—FIG. 2

As shown in FIG. 2, a patterned cover polyimide layer 14 having opening15 is formed over the Al bond pad 12, exposing a portion 19 of the Albond pad 12. Cover layer 14 is preferably a polyimide/benzocyclobutene(BCB) stack as will be used hereafter for purposes of illustration.

Polyimide/benzocyclobutene (BCB) layer opening 15 includes exposed sidewalls 11. The polyimide/benzocyclobutene (BCB) layer 14 may be patternedto form the opening 15 by, for example, photolithography and etching.The patterned polyimide/benzocyclobutene (BCB) layer opening 15 has awidth of preferably from about 30 to 90 μm and more preferably fromabout 30 to 60 μm.

Polyimide/benzocyclobutene (BCB)layer 14 has a combined thickness ofpreferably from about 5.0 to 10.0 μm and more preferably from about 5.0to 6.0 μm.

Formation of Additional Metal Layer 16—FIG. 3

As shown in FIG. 3, an additional metal layer 16 is formed over the Albond pad 12, polyimide/benzocyclobutene (BCB) layer 14 and the sidewalls 11 of polyimide/benzocyclobutene (BCB) layer 14 to a thickness ofpreferably from about 0.5 to 1.0 μm and more preferably from about 0.8to 1.0 μm. The additional metal layer 16 is preferably formed bysputtering.

Additional metal layer 16 is preferably comprised of aluminum (Al) orAlSi and is more preferably aluminum (Al) as will be used forillustrative purposes hereafter. Additional metal layer 16 is preferablycomprised of the same metal as is the metal bond pad 12.

Patterning of Additional Al Layer 16—FIG. 4

As shown in FIG. 4, the additional Al layer 16 is patterned to form apatterned additional Al layer/Al cap layer 16′ that at least covers theAl bond pad 12 and the side walls 11 of polyimide/benzocyclobutene (BCB)layer 14.

Double Zincation Process Activated Surface 19 and Electroless Nickel18/Immersion Gold 20 Layers Over Al Cap Layer 16′—FIG. 5

As shown in FIG. 5, the Al cap layer 16′ is subjected to a doublezincation process to form:

a double zincation activated surface 19 on Al cap layer 16′;

an electroless nickel layer 18 over the double zincation activatedsurface 19 of Al cap layer 16′ to a thickness of preferably from about4.8 to 5.2 μm and more preferably about 5.0 μm; and

an immersion gold (Au) layer over the electroless nickel layer 18 to athickness of preferably from about 0.09 to 0.11 μm and more preferablyabout 0.10 μm.

Formation of Rounded Solder Bump 22—FIG. 6

As shown in FIG. 6, a solder bump is formed over the immersion goldlayer 20 and is then subjected to a reflow process to form roundedsolder bump 22. Rounded solder bump 22 is preferably comprised of a tinlead alloy (SnPb), a tin silver copper alloy (SnAgCu), a tin silveralloy (SnAg) or a tin copper (SnCu) and is more preferably a tin silvercopper alloy (SnAgCu).

It is noted that due to the formation of the Al cap layer 16′ which issubjected to double zincation activated process to form: a doublezincation active Al surface 19; an electroless nickel layer 18; and animmersion gold layer 20; there is no electroless nickel layer18/immersion gold layer 20—polyimide interface so that the Al bond pad12 is not subject to corrosion. Further, good adhesion between the Albond pad/Al cap layer 16′ and the polyimide is achieved, leading toimproved reliability of the package.

The rounded solder bump 22 will come in contact with polyimide layer14/BCB only at the edge of the under bump metal (UBM).

Advantages of the Invention

The advantages of one or more embodiments of the present inventioninclude:

1) increased thickness of Al bond pad which could otherwise lead toserious pitting during the double zincation process; and

2) increased standoff (overall height) of solder bump thus improvingreliability performance.

While particular embodiments of the present invention have beenillustrated and described, it is not intended to limit the invention,except as defined by the following claims.

1. A method of forming a solder bump, comprising the steps of: providinga structure; forming a metal bond pad on the structure; forming apatterned cover layer over the structure; the patterned cover layerincluding an opening exposing a portion of the metal bond pad; thepatterned cover layer opening including side walls; the patterned coverlayer being comprised of a polyimide/benzocyclobutene stack; forming ametal cap layer over at least the exposed portion of the metal bond padand the patterned cover layer side walls; and forming a solder bump overthe metal cap layer.
 2. The method of claim 1, wherein the structure isa semiconductor wafer.
 3. The method of claim 1, wherein the structureis comprised of silicon or germanium.
 4. The method of claim 1, whereinthe metal bond pad is comprised of aluminum or AlSi; the metal cap layeris comprised of aluminum or AlSi; and the solder bump is comprised of atin lead alloy (SnPb), a tin silver copper alloy (SnAgCu), a tin silveralloy (SnAg) or a tin copper (SnCu).
 5. The method of claim 1, whereinthe metal bond pad is comprised of aluminum; the metal cap layer iscomprised of aluminum; and the solder bump is comprised of a tin silvercopper alloy (SnAgCu).
 6. The method of claim 1, wherein the metal bondpad and the metal cap layer are each comprised of the same metal.
 7. Themethod of claim 1, wherein the metal bond pad has a thickness of fromabout 0.5 to 1.5 μm; the patterned cover layer has a thickness of fromabout 5.0 to 10.0 μm; and the metal cap layer has a thickness of fromabout 0.5 to 1.0 μm.
 8. The method of claim 1, wherein the metal bondpad has a thickness of from about 1.0 to 1.5 μm; the patterned coverlayer has a thickness of from about 5.0 to 6.0 μm; and the metal caplayer has a thickness of from about 0.8 to 1.0 μm.
 9. The method ofclaim 1, wherein the patterned cover layer opening has a width of fromabout 30 to 90 μm.
 10. The method of claim 1, wherein the patternedcover layer opening has a width of from about 30 to 60 μm.
 11. Themethod of claim 1, wherein the metal cap layer is formed by sputtering.12. The method of claim 1, including the step of: subjecting the metalcap layer to a double zincation process.
 13. The method of claim 1,including the step of subjecting the metal cap layer to a doublezincation process to form: a double zincation activated surface on themetal cap layer, an electroless nickel layer on the double zincationactivated surface; and an immersion gold layer on the electroless nickellayer.
 14. The method of claim 1, including the step of subjecting themetal cap layer to a double zincation process to form: a doublezincation activated surface on the metal cap layer; an electrolessnickel layer on the double zincation activated surface; the electrolessnickel layer having a thickness of about 4.8 to 5.2 μm; and an immersiongold layer on the electroless nickel layer; the immersion gold layerhaving a thickness of about 0.09 to 0.11 μm.
 15. The method of claim 1,including the step of subjecting the metal cap layer to a doublezincation process to form: a double zincation activated surface on themetal cap layer; an electroless nickel layer on the double zincationactivated surface; the electroless nickel layer having a thickness ofabout 5.0 μm; and an immersion gold layer on the electroless nickellayer; the immersion gold layer having a thickness of about 0.10 μm. 16.The method of claim 1, including the step of reflowing the solder bumpto form a rounded solder bump.
 17. A method of forming a solder bump,comprising the steps of: providing a structure; forming a metal bond padon the structure; forming a patterned cover layer over the structure;the patterned cover layer including an opening exposing a portion of themetal bond pad; the patterned cover layer opening including side walls;the patterned cover layer being comprised of apolyimide/benzocyclobutene stack; forming a metal cap layer over atleast the exposed portion of the metal bond pad and the patterned coverlayer side walls; subjecting the metal cap layer to a double zincationprocess; and forming a solder bump over the metal cap layer.
 18. Themethod of claim 17, wherein the structure is a semiconductor wafer. 19.The method of claim 17, wherein the structure is comprised of silicon orgermanium.
 20. The method of claim 17, wherein the metal bond pad iscomprised of aluminum or AlSi; the metal cap layer is comprised ofaluminum or AlSi; and the solder bump is comprised of a tin lead alloy(SnPb), a tin silver copper alloy (SnAgCu), a tin silver alloy (SnAg) ora tin copper (SnCu).
 21. The method of claim 17, wherein the metal bondpad is comprised of aluminum; the metal cap layer is comprised ofaluminum; and the solder bump is comprised of a tin silver copper alloy(SnAgCu).
 22. The method of claim 17, wherein the metal bond pad and themetal cap layer are each comprised of the same metal.
 23. The method ofclaim 17, wherein the metal bond pad has a thickness of from about 0.5to 1.5 μm; the patterned cover layer has a thickness of from about 5.0to 10.0 μm; and the metal cap layer has a thickness of from about 0.5 to1.0 μm.
 24. The method of claim 17, wherein the metal bond pad has athickness of from about 1.0 to 1.5 μm; the patterned cover layer has athickness of from about 5.0 to 6.0 μm; and the metal cap layer has athickness of from about 0.8 to 1.0 μm.
 25. The method of claim 17,wherein the patterned cover layer opening has a width of from about 30to 90 μm.
 26. The method of claim 17, wherein the patterned cover layeropening has a width of from about 30 to 60 μm.
 27. The method of claim17, wherein the metal cap layer is formed by sputtering.
 28. The methodof claim 17, wherein the subjection of the metal cap layer to a doublezincation process forms: a double zincation activated surface on themetal cap layer; an electroless nickel layer on the double zincationactivated surface; and an immersion gold layer on the electroless nickellayer.
 29. The method of claim 17, wherein the subjection of the metalcap layer to a double zincation process forms: a double zincationactivated surface on the metal cap layer; an electroless nickel layer onthe double zincation activated surface; the electroless nickel layerhaving a thickness of from about 4.8 to 5.2 μm; and an immersion goldlayer on the electroless nickel layer; the immersion gold layer having athickness of from about 0.09 to 0.11 μm.
 30. The method of claim 17,wherein the subjection of the metal cap layer to a double zincationprocess forms: a double zincation activated surface on the metal caplayer; an electroless nickel layer on the double zincation activatedsurface; the electroless nickel layer having a thickness of about 5.0μm; and an immersion gold layer on the electroless nickel layer; theimmersion gold layer having a thickness of about 0.10 μm.
 31. The methodof claim 17, including the step of reflowing the solder bump to form arounded solder bump.
 32. A method of forming a solder bump, comprisingthe steps of: providing a structure; forming a metal bond pad on thestructure; forming a patterned cover layer over the structure; thepatterned cover layer including an opening exposing a portion of themetal bond pad; the patterned cover layer opening including side walls;the patterned cover layer being comprised of apolyimide/benzocyclobutene stack; forming a metal cap layer over atleast the exposed portion of the metal bond pad and the patterned coverlayer side walls; subjecting the metal cap layer to a double zincationprocess to form: a double zincation activated surface on the metal caplayer; an electroless nickel layer on the double zincation activatedsurface; and an immersion gold layer on the electroless nickel layer;and forming a solder bump over the immersion gold layer.
 33. The methodof claim 32, wherein the structure is a semiconductor wafer.
 34. Themethod of claim 32, wherein the structure is comprised of silicon orgermanium.
 35. The method of claim 32, wherein the metal bond pad iscomprised of aluminum or AlSi; the metal cap layer is comprised ofaluminum or AlSi; and the solder bump is comprised of a tin lead alloy(SnPb), a tin silver copper alloy (SnAgCu), a tin silver alloy (SnAg) ora tin copper (SnCu).
 36. The method of claim 32, wherein the metal bondpad is comprised of aluminum; the metal cap layer is comprised ofaluminum; and the solder bump is comprised of a tin silver copper alloy(SnAgCu).
 37. The method of claim 32, wherein the metal bond pad and themetal cap layer are each comprised of the same metal.
 38. The method ofclaim 32, wherein the metal bond pad has a thickness of from about 0.5to 1.5 μm; the patterned cover layer has a thickness of from about 5.0to 10.0 μm; and the metal cap layer has a thickness of from about 0.5 to1.0 μm.
 39. The method of claim 32, wherein the metal bond pad has athickness of from about 1.0 to 1.5 μm; the patterned cover layer has athickness of from about 5.0 to 6.0 μm; and the metal cap layer has athickness of from about 0.8 to 1.0 μm.
 40. The method of claim 32,wherein the patterned cover layer opening has a width of from about 30to 90 μm.
 41. The method of claim 32, wherein the patterned cover layeropening has a width of from about 30 to 60 μm.
 42. The method of claim32, wherein the metal cap layer is formed by sputtering.
 43. The methodof claim 32, including the step of reflowing the solder bump to form arounded solder bump.
 44. The method of claim 32, wherein: theelectroless nickel layer has a thickness of from about 4.8 to 5.2 μm;and the immersion gold layer having a thickness of from about 0.09 to0.11 μm.
 45. The method of claim 32, wherein: the electroless nickellayer has a thickness of about 5.0 μm; and the immersion gold layerhaving a thickness of about 0.10 μm.